Title: SERPENTINIZATION OF ULTRAMAFIC ROCKS FROM ITI AND KALLIDROMON MOUNTAINS (CENTRAL GREECE)
Abstract: Variably serpentinized peridotites occur in the ophiolite melanges and the remnant nappes of Iti and Kallidromon ophiolites. The serpentinization extents between 80 and 100% in the harzburgite and 30-80% in the lherzolite; the dunite is slightly serpentinized. The studied rocks comprise mainly serpentine and minor chlorite, tremolite, talc and calcite; accessory pentlandite, native copper and millerite occur, too. Relic orthopyroxene, clinopyroxene, olivine and Cr- or Al-spinel (depending on the protolith), are also present.
Antigorite and lizardite, identified by XRD, display mesh, hourglass, interpenetrating, interlocking and ribbon textures. Olivine is replaced by serpentine in mesh textures, while orthopyroxene is replaced by bastite. The mesh cores consist of serpentine, isotropic serpentine, unaltered olivine and magnetite (Fig. 1a). In the mesh rims, fibers of lizardite are generally oriented perpendicular to the mesh cells. Bastite is made up of serpentine and scarce chlorite and tremolire fibers (Fig. 1b). In the type-2 lizardite hourglass texture, the mesh rim seems to extend to the center of the cell (Fig. 1c). Interpenetrating texture consists of elongated, interpenetrating blades of antigorite (Fig. 1d), while interlocking texture consists of more equant grains of lizardite and antigorite (Fig. 1e). In both the interpenetrating and interlocking textures, antigorite replaces lizardite. Locally, lizardite display ribbon texture (Fig. 1f). In the serpentinized harzburgites, the Cr-spinels are altered to ferritchromit and magnetite and are surrounded by Cr-chlorite (Fig. 1g).
Unlike the bastitic serpentine, with considerable Al and Cr abundances, the mesh serpentine has low Al and Cr, but in turn it contains some Ni, matching the composition of olivine. In general, the microtextural site where the serpentine has crystallized clearly influences its chemistry (e.g. Mevel and Stamoudi 1996, O’Hanley 1996, Puga et al. 1999). Si and Mg increase, while Fet decreases from core to the rim, in the mesh serpentine. Isotropic serpentine analyzed in mesh cores, is almost devoid of Al and contains Fe2O3<2%. The ribbon serpentine resembles the mesh serpentine.
The tremolite analysed in the serpentinites differs from metasomatic tremolite occurring in the least altered peridotites (Karipi et al. 2005), by having lower Si and higher Al. The chlorite that surrounds the altered Cr-spinels contains 2-3% Cr and is related to liberation of Cr, Al and Mg from the Cr-spinel during its alteration to ferritchromit (e.g. Fleet et al. 1993; Christofides et al. 1994, Michailidis 1995). According to O’Hanley (1996), the formation of Cr-chlorite during serpentinization depends on the compositions of Crspinel and pyroxene and the conditions under which they breakdown.
Serpentine minerals are stable at temperatures between 500°C and 20°C and are formed generally at P<2 kb (O’Hanley 1996, Mevel 2003), via reactions relatively insensitive to pressure. The Iti and Kallidromon ophiolites are of SSZ origin (Karipi et al. 2005), a regime that favours their serpentinization to be related to seawater, as well as to fluids released from the subducted oceanic crust and sediments. The presence of antigorite and lizardite and the absence of metamorphic forsterite provide an upper thermal limit for the serpentinization, at around 350-370oC (P<2kb), indicated by the reaction (1) (Fig. 2). In such a case, the lizardite has been preserved as a metastable phase (Fig. 2). The presence of talc, which is stable above 420-450oC, in the serpentinites suggests high silica activity in the fluid phase (Hebert et al. 1990). Moreover, experimental studies at 400°C (Allen and Seyfried 2003) suggest that the preferential alteration of pyroxenes (with respect to olivine) may result in fluids enriched in dissolved Ca, SiO2, Fe and H+, which favour the formation of Si-rich silicates (talc and tremolite) and results in their stability over serpentine. According to Manning (1996) and Peacock and Hyndman (1999), talc may form in the forearc mantle along the plate interface where infiltrating fluids derived from the underlying subducting crust are silica-saturated and mechanical mixing of mantle with siliceous sediments may occur. In the studied serpentinites, the opaque assemblage of magnetite + pentlandite ± millerite indicates low ƒO2 which may attributed to the production of H+ during the formation of magnetite (Alt and Shanks 1998).
Publication Year: 2005
Publication Date: 2005-01-07
Language: en
Type: article
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